The successor state of a state (Q,Qf) with input σ is defined as (sucσ∃(Q),sucσ∃(Q)∩F) if Qf={}, and as (sucσ∃(Q),sucσ∃(Qf)∩F) otherwise.
In case of state q2=({s2,s3},{s3}) and input b, we therefore get
q3 := (sucσ∃(Q),sucσ∃(Qf)∩F)
= (sucσ∃({s2,s3}),sucσ∃({s3})∩{s3})
= ({s2,s3},{}∩{s3})
= ({s2,s3},{})